Thymus derived lymphocytes (T cells) ar a major component of the gut associated lymphoid tissue (GALT) and may play a prominent role in the pathogenesis of a number of intestinal disorders. These include such diseases as inflammatory bowel disease, infectious enteritides and graft-versus-host disease in which the number of T lymphocytes are increased and those present are activated. In order to gain an insight into these disorders, it is important to understand the fundamental mechanisms of T cell function. A T cell recognizes processed nominal antigens in association with the components of the major histocompatibility complex (MHC) on the surface of a target cell via the T cell receptor (TCR). The TCR, which subserves both antigen and MHC recognition, is composed of two immunoglobulin like proteins, alpha and beta or omega and delta. Noncovalently associated with the TCR are the nonvariant chains of the CD3 complex; CD3 omega, delta, epsilon, and , which may be important in transmembrane signaling after the TCR binds antigen/MHC. In order for a T cell to respond to antigen/MHC it must correctly assemble the polypeptide chains of the TCR/CD3 complex and transport them to the cell surface. Assembly of the protein of TCR/CD3 complex and functional competence of the T cell are, therefore, closely coupled. The specific aims of this study are to (1) through the use of transfection studies with specific cDNA's in T cell mutants and non-T cells, delineate the protein chains required for assembly and surface expression of the TCR/CD3 complex; (2) to define structural domains important in assembly and surface expression by mutating cDNA's in vitro with site directed and saturation mutagenesis and transfecting the altered cDNA's into T cells; (3) to define structural domains important in signal transduction by mutating cDNA's in vitro with site directed and saturation mutagenesis as well as with specific restriction endonucleases and transfecting the altered cDNA's into T cells and (4) characterize the functional defect in T cell activation mutants by transfection of normal cDNA's into the T cell mutants and cloning and sequencing the abnormal cDNA's. Under the direction of Dr. Cox Terhorst, these studies will provide the investigator with an extensive experience in molecular immunology. This knowledge can, in the future, be directly applied as an independent academic investigator to studies of T cell differentiation and activation in the GALT.